Photodiode

ABSTRACT

A photodiode according to example embodiments includes an anode, a cathode, and an intrinsic layer between the anode and the cathode. The intrinsic layer includes a P-type semiconductor and an N-type semiconductor, and composition ratios of the P-type semiconductor and the N-type semiconductor vary within the intrinsic layer depending on a distance of the intrinsic layer from one of the anode and the cathode.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.13/354,980 filed on Jan. 20, 2012, which claims priority to and thebenefit of Korean Patent Application No. 10-2011-0092085 filed in theKorean Intellectual Property Office on Sep. 9, 2011, the entire contentsof each of which are incorporated herein by reference.

BACKGROUND 1. Field

Example embodiments relate to a photodiode including an organicsemiconductor.

2. Description of the Related Art

The resolution of an image sensor including a photodiode becomes higher,and thus, the size of a pixel becomes decreased. The reduction of thepixel size may cause the decrease of the absorption area that in turnreduces the sensitivity of a silicon photodiode.

Therefore, organic semiconductors having a higher extinction coefficientand higher wavelength selectivity compared with silicon are beingconsidered as photoelectric materials of a photodiode.

A photodiode including an organic semiconductor as a photoelectricmaterial generally has a triple-layered structure that includes a P-typesemiconductor, an intrinsic layer, and an N-type semiconductor. Theintrinsic layer is formed by co-depositing a P-type semiconductor and anN-type semiconductor. The intrinsic layer absorbs light to produceexcitons, and the excitons are divided into holes and electrons at ajoint surface of the N-type semiconductor and the P-type semiconductor.The holes and electrons move to electrodes to generate current. However,the external quantum efficiency and the light responsivity of theabove-described photodiode may not be desirable.

SUMMARY

According to example embodiments, a photodiode may include an anode, acathode, and an intrinsic layer between the anode and the cathode. Theintrinsic layer may include a P-type semiconductor and an N-typesemiconductor. Composition ratios of the P-type semiconductor and theN-type semiconductor may vary within the intrinsic layer depending on aposition.

Composition ratios of the P-type semiconductor and the N-typesemiconductor may vary within the intrinsic layer depending on adistance of the intrinsic layer from one of the anode and the cathode.

A composition ratio of the P-type semiconductor in the intrinsic layermay increase closer to the anode, and a composition ratio of the N-typesemiconductor in the intrinsic layer may increase closer to the cathode.A variation of the composition ratios of the P-type semiconductor andthe N-type semiconductor in the intrinsic layer may be continuous in adirection from the anode to the cathode.

The intrinsic layer may include at least two sublayers deposited insequence, the at least two sublayers having different compositionratios. The at least two sublayers may include a first sublayer closestto the anode, the first sublayer having a greater composition ratio ofthe P-type semiconductor than the composition ratio of the N-typesemiconductor, and a second sublayer closest to the cathode, the secondsublayer having a greater composition ratio of the N-type semiconductorthan the composition ratio of the P-type semiconductor.

A composition ratio of the P-type semiconductor in the at least twosublayers may increase closer to the anode, and a composition ratio ofthe N-type semiconductor in the at least two sublayers may increasecloser to the cathode. The composition ratio of the P-type semiconductorrelative to the N-type semiconductor in the first sublayer may begreater than about 1 and smaller than about 1,000, and the compositionratio of the P-type semiconductor relative to the N-type semiconductorin the second sublayer may be smaller than about 1 and greater thanabout 1/1,000.

The at least two sublayers may further include a third sublayer betweenthe first sublayer and the second sublayer. A composition ratio of theP-type semiconductor relative to the N-type semiconductor may be about10 in the first sublayer, about 1/10 in the second sublayer, and about 5in the third sublayer.

Each of the at least two sublayers may have a thickness of about 1 nm toabout 100 nm. The P-type semiconductor of the intrinsic layer mayinclude N,N′-dimethyl quinacridone (DMQA), and the N-type semiconductorof the intrinsic layer may include at least one of C60, C70, and[6,6]-phenyl-C61-butyric acid methyl ester (PCBM). The N-typesemiconductor of the intrinsic layer may include 060. The photodiode mayfurther include an N-type semiconductor layer between the intrinsiclayer and the cathode, the N-type semiconductor layer substantially notincluding a P-type semiconductor.

The photodiode may further include an electron blocking layer betweenthe intrinsic layer and the anode, the electron blocking layer includingat least one of poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate)(PEDOT:PSS), 4,4′,4″-tris(N-(2-naphthyl)-N-phenyl-amino)-triphenylamine(2TNATA), a molybdenum oxide, and a zinc oxide.

The photodiode may further include a P-type semiconductor layer betweenthe intrinsic layer and the anode, the P-type semiconductor layer notincluding an N-type semiconductor.

The photodiode may further include a hole blocking layer between theintrinsic layer and the cathode, the hole blocking layer including atleast one of 4,7-diphenyl-1,10-phenanthroline (Bphen), Benocyclidine(BCP), and 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBI).

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects will become apparent and more readilyappreciated from the following description of example embodiments, takenin conjunction with the accompanying drawings in which:

FIGS. 1-7 are schematic sectional views of photodiodes according toexample embodiments.

FIG. 8 is a schematic sectional view of an experimental example of aphotodiode according to example embodiments.

FIG. 9 is a schematic sectional view of a comparative example of aphotodiode.

FIG. 10 is a graph showing external quantum efficiency (EQE) of theexperimental and comparative examples of photodiodes as a function ofwavelength of incident light.

FIG. 11 is a graph showing photo current density of the experimental andcomparative examples of photodiodes as a function of illumination.

DETAILED DESCRIPTION

Example embodiments will be described more fully hereinafter withreference to the accompanying drawings. As those skilled in the artwould realize, the described embodiments may be modified in variousdifferent ways, all without departing from the spirit or scope. In thedrawing, parts having no relationship with the explanation are omittedfor clarity, and the same or similar reference numerals designate thesame or similar elements throughout the specification.

It will be understood that when an element is referred to as being“connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present. As used herein the term “and/or” includesany and all combinations of one or more of the associated listed items.

It will be understood that, although the terms “first”, “second”, etc.may be used herein to describe various elements, components, regions,layers and/or sections, these elements, components, regions, layersand/or sections should not be limited by these terms. These terms areonly used to distinguish one element, component, region, layer orsection from another element, component, region, layer or section. Thus,a first element, component, region, layer or section discussed belowcould be termed a second element, component, region, layer or sectionwithout departing from the teachings of example embodiments.

Spatially relative terms, such as “beneath,” “below,” “lower,” “above,”“upper” and the like, may be used herein for ease of description todescribe one element or feature's relationship to another element(s) orfeature(s) as illustrated in the figures. It will be understood that thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. For example, if the device in thefigures is turned over, elements described as “below” or “beneath” otherelements or features would then be oriented “above” the other elementsor features. Thus, the exemplary term “below” can encompass both anorientation of above and below. The device may be otherwise oriented(rotated 90 degrees or at other orientations) and the spatially relativedescriptors used herein interpreted accordingly.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of exampleembodiments. As used herein, the singular forms “a,” “an” and “the” areintended to include the plural forms as well, unless the context clearlyindicates otherwise. It will be further understood that the terms“comprises” and/or “comprising,” when used in this specification,specify the presence of stated features, integers, steps, operations,elements, and/or components, but do not preclude the presence oraddition of one or more other features, integers, steps, operations,elements, components, and/or groups thereof.

Example embodiments are described herein with reference tocross-sectional illustrations that are schematic illustrations ofidealized embodiments (and intermediate structures) of exampleembodiments. As such, variations from the shapes of the illustrations asa result, for example, of manufacturing techniques and/or tolerances,are to be expected. Thus, example embodiments should not be construed aslimited to the particular shapes of regions illustrated herein but areto include deviations in shapes that result, for example, frommanufacturing. For example, an implanted region illustrated as arectangle will, typically, have rounded or curved features and/or agradient of implant concentration at its edges rather than a binarychange from implanted to non-implanted region. Likewise, a buried regionformed by implantation may result in some implantation in the regionbetween the buried region and the surface through which the implantationtakes place. Thus, the regions illustrated in the figures are schematicin nature and their shapes are not intended to illustrate the actualshape of a region of a device and are not intended to limit the scope ofexample embodiments.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which example embodiments belong. Itwill be further understood that terms, such as those defined incommonly-used dictionaries, should be interpreted as having a meaningthat is consistent with their meaning in the context of the relevant artand will not be interpreted in an idealized or overly formal senseunless expressly so defined herein.

Photodiodes according to example embodiments are described in detailwith reference to the drawings. In this regard, example embodiments mayhave different forms and should not be construed as being limited to thedescriptions set forth herein. Accordingly, example embodiments aremerely described below, by referring to the figures, to explain aspectsof the present description.

FIG. 1 and FIG. 2 are schematic sectional views of photodiodes accordingto example embodiments. Referring to FIG. 1, a photodiode 100 accordingto example embodiments includes an intrinsic layer 110, and an anode 120and a cathode 130 disposed on opposite sides of the intrinsic layer 110.Although FIG. 1 shows that the anode 120 is disposed under the intrinsiclayer 110 and the cathode 130 is disposed on the intrinsic layer 110,the cathode 130 may be disposed under the intrinsic layer 110 and theanode 120 may be disposed on the intrinsic layer 110.

The intrinsic layer 110 includes both a P-type semiconductor and anN-type semiconductor, and the composition ratio of the P-typesemiconductor and the N-type semiconductor may vary depending on theposition of the intrinsic layer 110. For example, the composition ratioof the P-type semiconductor may be greater than that of the N-typesemiconductor at a position closer to the anode 120, while thecomposition ratio of the N-type semiconductor may be greater than thatof the P-type semiconductor at a position closer to the cathode 130. Inaddition, the composition ratios of the P-type semiconductor and theN-type semiconductor may be different at different positions, thedistances from which to the anode 120 are substantially equal to eachother, for example, at different positions that have substantially thesame height in FIG. 1.

In the intrinsic layer 110, the composition ratios of the P-typesemiconductor and the N-type semiconductor may vary continuously. Forexample, the composition ratios of the P-type semiconductor and theN-type semiconductor in the intrinsic layer 110 may vary gradually fromabout 1000:1 to about 1:1000 from the anode 120 to the cathode 130.However, the composition ratios of the P-type semiconductor and theN-type semiconductor are not limited thereto.

The composition ratios of the P-type semiconductor and the N-typesemiconductor in the intrinsic layer 110 may change stepwise. In exampleembodiments, the intrinsic layer 110 may include two or more sublayershaving different composition ratios, and a sublayer closer to the anode120 may have a higher concentration of the P-type semiconductor and alower concentration of the N-type semiconductor, while another sublayercloser to the cathode 130 may have a lower concentration of the P-typesemiconductor and a higher concentration of the N-type semiconductor.

For example, the P-type semiconductor may include N,N′-dimethylquinacridone (DMQA), and the N-type semiconductor may include at leastone of C60, C70, and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM).However, a variety of other semiconductive materials may also be used.

The intrinsic layer 110 may be formed by co-depositing the P-typesemiconductor and the N-type semiconductor by means of thermalevaporation. However, the deposition method may not be limited thereto.

The anode 120 may include a transparent conductive material, e.g.,indium-tin-oxide (ITO) and indium-zinc-oxide (IZO), so that light maypass therethrough, but the material for the anode 120 may not be limitedthereto. The cathode 130 may include a metal, e.g., Al, but the materialfor the cathode 130 may not be limited thereto.

The anode 120 may be formed by sputtering, and the cathode 130 may beformed by thermal evaporation. However, the deposition methods for theanode 120 and the cathode 130 may not be limited thereto.

Referring to FIG. 2, a photodiode 200 according to example embodimentsincludes an anode 220, a cathode 230, and an intrinsic layer 210disposed between the anode 220 and the cathode 230. The intrinsic layer210 may include three sublayers 212, 214, and 216.

Each of the sublayers 212, 214, and 216 includes a P-type semiconductorand an N-type semiconductor. The composition ratios of the P-typesemiconductor and the N-type semiconductor may be different in thesublayers 212, 214, and 216. The composition ratio of the P-typesemiconductor may increase, and the composition ratio of the N-typesemiconductor may decrease from the anode 220 to the cathode 230. Forexample, the composition ratio of the P-type semiconductor may be thegreatest at the sublayer 212 that is closest to the anode 220, thesmallest at the sublayer 216 that is closest to the cathode 230, andintermediate at the intermediate sublayer 214. Contrary to the P-typesemiconductor, the composition ratio of the N-type semiconductor may bethe greatest at the sublayer 216 that is closest to the cathode 230, thesmallest at the sublayer 212 that is closest to the anode 220, andintermediate at the intermediate sublayer 214.

Furthermore, the composition ratio of the P-type semiconductor may begreater than that of the N-type semiconductor in the sublayer 212 thatis close to the anode 220, and the composition ratio of the N-typesemiconductor may be greater than that of the P-type semiconductor inthe sublayer 216 that is close to the cathode 230. The composition ratioof the P-type semiconductor may be equal to or greater than the N-typesemiconductor in the intermediate sublayer 214, or vice versa.

The composition ratio of the P-type semiconductor (CRP) relative to thatof the N-type semiconductor (CRN), that is, CRP/CRN in the sublayer 212that is closer to the anode 220 may be greater than about one andsmaller than about 1,000, while the CRP/CRN may be smaller than aboutone and greater than about 1/1,000 in the sublayer 216 that is closer tothe cathode 230.

In each of the sublayers 212, 214, and 216, the composition ratios ofthe P-type semiconductor and the N-type semiconductor may vary dependingon the position or the height. The thickness of each sublayer 212, 214,or 216 may be from about 1 nm to about 100 nm. The materials and formingmethods of the layers shown in FIG. 2 may be substantially the same asthose in FIG. 1. Although the number of the sublayers 212, 214, 216shown in FIG. 2 is three, the number may be equal to two, four or more.

Incident light may enter from the transparent anode 120 or 220 of thephotodiode shown in FIG. 1 or FIG. 2, and the intrinsic layer 110 or 210may absorb a light with a predetermined or given wavelength to produceexcitons therein. The excitons may be divided into holes and electronsat the joint surface between the N-type semiconductor and the P-typesemiconductor in the intrinsic layer 110 or 210. The holes may movetoward the anode 120 or 220, while the electrons may move toward thecathode 130 or 230 so that the current may be generated in thephotodiode 100 or 200.

The higher concentration of the P-type semiconductor in the intrinsiclayer 110 or 210 at a portion near the anode 120 or 220 may facilitatethe holes generated at the portion to escape to the nearby anode 120 or220. Similarly, the electrons generated at a portion that is close tothe cathode 130 or 230 may easily escape to the cathode 130 or 230because the composition ratio of the N-type semiconductor at the portionis higher. Therefore, the response time of the photodiode 100 or 200 forthe incident light may be shortened.

FIG. 3 to FIG. 7 are schematic sectional views of photodiodes accordingto example embodiments. Referring to FIG. 3, a photodiode 300 accordingto example embodiments includes an intrinsic layer 310, an anode 320 anda cathode 330 that are disposed on opposite sides of the intrinsic layer310, and an N-type layer 340 disposed between the intrinsic layer 310and the cathode 330. The intrinsic layer 310, the anode 320, and thecathode 330 may be substantially the same as those shown in FIG. 1 orFIG. 2.

The N-type layer 340 includes an N-type semiconductor but not a P-typesemiconductor, and the N-type semiconductor in the N-type layer 340 maybe the same as that in the intrinsic layer 310. The N-type layer, forexample, C60 having higher electron mobility, may contribute to therelatively smooth performance of the photodiode 300 without a P-typelayer (not shown) that has lower mobility.

Referring to FIG. 4, a photodiode 400 according to example embodimentsincludes an intrinsic layer 410, an anode 420 and a cathode 430 that aredisposed on opposite sides of the intrinsic layer 410, and a P-typelayer 450 disposed between the intrinsic layer 410 and the anode 420.The intrinsic layer 410, the anode 420, and the cathode 430 may besubstantially the same as those shown in FIG. 1 or FIG. 2.

The P-type layer 450 includes a P-type semiconductor but not an N-typelayer, and the P-type semiconductor in the P-type layer 450 may be thesame as the P-type semiconductor in the intrinsic layer 410.

Referring to FIG. 5, a photodiode 500 according to example embodimentsincludes an intrinsic layer 510, an anode 520 and a cathode 530 that aredisposed on opposite sides of the intrinsic layer 510, an N-type layer540 disposed between the intrinsic layer 510 and the cathode 530, and aP-type layer 550 disposed between the intrinsic layer 510 and the anode520. The intrinsic layer 510, the anode 520, and the cathode 530 may besubstantially the same as those shown in FIG. 1 or FIG. 2.

The N-type layer 540 includes an N-type semiconductor but not a P-typesemiconductor, and the N-type semiconductor in the N-type layer 540 maybe the same as that in the intrinsic layer 510. The P-type layer 550includes a P-type semiconductor but not an N-type layer, and the P-typesemiconductor in the P-type layer 550 may be the same as the P-typesemiconductor in the intrinsic layer 510.

Referring to FIG. 6, a photodiode 600 according to example embodimentsincludes an intrinsic layer 610, an anode 620 and a cathode 630 that aredisposed on opposite sides of the intrinsic layer 610, an N-type layer640 disposed between the intrinsic layer 610 and the cathode 630, and anelectron blocking layer 660 disposed between the intrinsic layer 610 andthe anode 620. The intrinsic layer 610, the anode 620, and the cathode630 may be substantially the same as those shown in FIG. 1 or FIG. 2,and the N-type layer 640 may be substantially the same as that shown inFIG. 3.

The electron blocking layer 660 may be referred to as the hole transportlayer, and may block electrons from moving from the anode 620 to theintrinsic layer 610, thereby accelerating the light absorbance of theintrinsic layer 610 to increase the production of excitons. The electronblocking layer 660 may include at least one of organic materials, e.g.,poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS),4,4′,4″-tris(N-(2-naphthyl)-N-phenyl-amino)-triphenylamine (2TNATA), andinorganic materials, e.g., molybdenum oxides and zinc oxides. The N-typelayer 640 may be removed.

Referring to FIG. 7, a photodiode 700 according to example embodimentsincludes an intrinsic layer 710, an anode 720 and a cathode 730 that aredisposed on opposite sides of the intrinsic layer 710, an N-type layer740 and a hole blocking layer 770 that are disposed in sequence betweenthe intrinsic layer 710 and the cathode 730, and a P-type layer 750 andan electron blocking layer 760 that are disposed in sequence between theintrinsic layer 710 and the anode 720. The intrinsic layer 710, theanode 720, and the cathode 730 may be substantially the same as thoseshown in FIG. 1 or FIG. 2, the N-type layer 740 may be substantially thesame as that shown in FIG. 3, the P-type layer 750 may be substantiallythe same as that shown in FIG. 4, and the electron blocking layer 760may be substantially the same as that shown in FIG. 6.

The hole blocking layer 770 may be referred to as the electron transportlayer, and may block holes from moving from the cathode 730 to theintrinsic layer 710, thereby enhancing the light absorbance of theintrinsic layer 710 to produce more excitons. The hole blocking layer770 may include at least one of 4,7-diphenyl-1,10-phenanthroline(Bphen), Benocyclidine (BCP), and1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBI). At least one ofthe N-type layer 740, the P-type layer 750, and the electron blockinglayer 760 may be abbreviated.

Experimental and comparative examples of photodiodes are described indetail with reference to FIG. 8 to FIG. 11. FIG. 8 is a schematicsectional view of an experimental example of a photodiode according toexample embodiments, FIG. 9 is a schematic sectional view of acomparative example of a photodiode, FIG. 10 is a graph showing externalquantum efficiency (EQE) of the experimental and comparative examples ofphotodiodes as function of wavelength of incident light, and FIG. 11 isa graph showing photo current density of the experimental andcomparative examples of photodiodes as function of illumination.

A photodiode 800 having a structure shown in FIG. 8 was manufactured.Referring to FIG. 8, an anode 820 having a thickness of about 100 nm wasformed by sputtering ITO, and an electron blocking layer 860 of about 30nm thickness was formed by spin-coating PEDOT:PSS.

DMQA as a P-type semiconductor and fullerene (C60) as an N-typesemiconductor were co-deposited by thermal evaporation to form anintrinsic layer 810. In detail, a lower sublayer 812, an intermediatesublayer 814, and an upper sublayer 816 were deposited in sequence byvarying the composition ratios of DMQA and C60. The composition ratioDMQA:C60 was about 10:1 in the lower sublayer 812, about 5:1 in theintermediate sublayer 814, and about 1:10 the upper sublayer 816. Thethicknesses of the lower sublayer 812, the intermediate sublayer 814,and the upper sublayer 816 were about 10 nm, about 30 nm, and about 10nm, respectively. C60 was thermally deposited to form an N-type layer840 having thickness of about 30 nm. A cathode 830 having a thickness ofabout 100 nm was formed by thermal evaporation of aluminum (Al).

A photodiode 900 having a structure shown in FIG. 9 was manufactured forcomparison. Referring to FIG. 9, an anode 920 having a thickness ofabout 100 nm was formed by sputtering ITO, and an electron blockinglayer 960 of about 30 nm thickness was formed by spin-coating PEDOT:PSS.

A P-type layer 950 having a thickness of about 30 nm was formed bythermal evaporation of DMQA as a P-type semiconductor, an intrinsiclayer 910 having a thickness of about 50 nm was formed by co-depositingDMQA as a P-type semiconductor and C60 as an N-type semiconductor with acomposition ratio of about 5:1 by means of thermal evaporation, and anN-type layer 940 having a thickness of about 30 nm was formed by thermalevaporation of C60. A cathode 930 having a thickness of about 100 nm wasformed by thermal evaporation of Al.

For the photodiodes 800 and 900 manufactured as described, externalquantum efficiency (EQE) and photo responsivity were measured.

As shown in FIG. 10 that illustrates the EQE of the photodiodes as afunction of wavelength of incident light, the experimental photodiode800 has higher EQE than the comparative photodiode 900 generally. Inparticular, at the wavelength of about 540 nm where the EQE is thehighest, the EQE of the experimental photodiode 800 is about 20%, whichis greater than that of the comparative photodiode 900 by about 3%,i.e., about 17%.

As shown in FIG. 11 that illustrates photo current density of thephotodiodes as a function of illumination, the responsivity of theexperimental photodiode 800 is improved compared to that of thecomparative photodiode 900.

While this disclosure has been described in connection with what ispresently considered to be practical example embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments, but, on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

What is claimed is:
 1. A photodiode comprising: an anode; a cathode; andan intrinsic layer between the anode and the cathode, the intrinsiclayer including lower, intermediate, and upper sublayers deposited insequence, wherein a composition ratio of a P-type semiconductor relativeto a N-type semiconductor is substantially constant throughout each ofthe lower, intermediate, and upper sublayers, wherein each sublayer ofthe lower, intermediate, and upper sublayers includes both of the P-typesemiconductor and the N-type semiconductor, wherein the compositionratio of the P-type semiconductor relative to the N-type semiconductorin the lower sublayer is an inverse of the composition ratio of theP-type semiconductor relative to the N-type semiconductor in the uppersublayer, wherein the composition ratio of the P-type semiconductorrelative to the N-type semiconductor in the intermediate sublayer isabout 5:1, and wherein the composition ratio of the P-type semiconductorrelative to the N-type semiconductor in the lower sublayer is greaterthan 10:1 and smaller than about 1000:1, and the composition ratio ofthe P-type semiconductor relative to the N-type semiconductor in theupper sublayer is smaller than 1:10 and greater than about 1:1000. 2.The photodiode of claim 1, wherein each of lower, intermediate, andupper sublayers has a thickness of about 1 nm to about 100 nm.
 3. Thephotodiode of claim 1, further comprising: an N-type semiconductor layerbetween the intrinsic layer and the cathode, the N-type semiconductorlayer substantially not including the P-type semiconductor.
 4. Thephotodiode of claim 3, further comprising: an electron blocking layerbetween the intrinsic layer and the anode, the electron blocking layerincluding at least one of poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT:PSS),4,4′,4″-tris(N-(2-naphthyl)-N-phenyl-amino)-triphenylamine (2TNATA), amolybdenum oxide, and a zinc oxide.
 5. The photodiode of claim 1,further comprising: a P-type semiconductor layer between the intrinsiclayer and the anode, the P-type semiconductor layer not including theN-type semiconductor.
 6. The photodiode of claim 1, further comprising:a hole blocking layer between the intrinsic layer and the cathode, thehole blocking layer including at least one of4,7-diphenyl-1,10-phenanthroline (Bphen), Benocyclidine (BCP), and1,3,5-tris(1-phenyl-1H-benziInidazol-2-yl)benzene (TPBI).